Heisler was born in Downham Market, Norfolk, England in 1941,[2] the son of Austrian migrants to the United Kingdom from Vienna in 1938.
His mother was raised as a Catholic in the Sudetenland, the daughter of the manager of a graphite mine who had been the first socialist in his village around 1890 or 1900.
He describes himself as having the classic background of a book collector, having suffered a serious illness at the age of 3 or 4 when he was in a coma and "given up for dead".
The family moved to London from Norfolk on the back of a potato truck arriving at 337 Victoria Park Road, Hackney, at 8.00 pm on Christmas Eve, 1947.
[3] Heisler describes himself as "becoming politicised" during the Suez Crisis of 1956 and became involved in the Young Socialists in Hackney Central in London where he was influenced by Robin Jamieson.